Field of the Invention
The invention relates to a device for detecting mis-fed or missing sheets in a turning device of a printing press.
Devices for detecting mis-fed or missing sheets in turning devices of printing presses have become known heretofore and serve to detect defects in sheet position which occur when a sheet is turned, and any other disruptions in sheet travel, respectively, and to interrupt the printing process if a mis-fed or missing sheet, such as a short sheet, appears, in order thereby to avert damage to the printing press or parts thereof. Short sheets are produced, for example, due to tolerances in length of the sheets being processed, which for uncut papers can be in the range of several millimeters and, as a result of these tolerances, during the turning operation, the sheets are not correctly gripped by the grippers which grip the trailing edge of the sheet and are therefore lost.
Because of the high printing speed at which sheet-fed rotary offset printing presses are currently run, even in recto/verso or first-form and perfector printing, it is of major importance that the timespan within which a mis-fed or missing sheet is detected and the printing press is shutoff be very short, so that damage to the printing press can be reliably precluded even at the highest printing speeds.
From the published German Patent Document DE-AS 26 21 250, a sheet detecting device of this general type has become known wherein, on the storage drum of a turning device of a sheet-fed rotary offset printing press, a first row of suction grippers is provided in the vicinity of the trailing edge of the sheet for catching and holding the trailing edge of the sheet by suction as the sheet is being turned. The suction grippers are connected to a first negative pressure generator via a first connecting line.
As viewed in the rotary direction of the storage drum, upstream of the row of suction grippers, there is arranged a further row of detecting nozzles connected to a second negative pressure generator via a second connecting line and a pressure switch fluidically connected to the connecting line. Because a separate negative pressure generator is provided, the detecting nozzles can operate at a low constant pressure. The detecting nozzles are covered by the trailing edge of the sheet, when a sheet of normal length is being transported properly; consequently, the negative pressure in the second connecting line is maintained, and the pressure switch is not actuated. In the case of a mis-fed or a missing or short sheet, caused by some problem in the sheet travel, the openings of the detecting nozzles are covered only partially, if at all, by the trailing edge of the sheet; consequently, a change in pressure occurs in the second connecting line and is detected by the pressure switch which causes the immediate discontinuation of the printing process or shutoff of the printing press. A disadvantage of the hereinaforedescribed device is that two separate negative pressure sources with different pressures are required in order to permit safe and reliable operation of the device.